The present invention relates to an additive composition for use in steel making. More particularly, the present invention relates to an additive composition which shows excellent activity in deoxidation, desulphurization and dephosphorization and makes slag to be of high fluidity. Also, the present invention is concerned with a method for making special steel superior in mechanical properties, by. use of the additive composition.
When making steel, lime (CaO) or fluorite (CaF2), amounting up to 10% by weight of the amount of the ore to be fed, is conventionally added in metal blast furnaces to remove impurities such as phosphorous (P) and sulfur (S) and to make the fluidity of slag better. Also, in order to give the mechanical properties required for the use of special steel, metal additives are frequently used. Such additives, however, play an incomplete role in removing he impurities from iron melts.
In the case of using the metal additives for alloy, their specific gravities are different from that of iron, the base material, so that an imbalance occurs upon formulation of metals. In addition, the metal components of the additives are often oxidized, which makes it more difficult to obtain desired steel products. After all, expensive, high purity metal additives are rising as an alternative, but give rise to an increase in the production cost.
The problems ascribed to the difference in specific gravity may be overcome by stirring the iron melts of metal blast furnaces at constant speeds, but this is extremely difficult. With the aim of avoiding the difference of specific gravity between iron components and alloy additive components in an iron melt, attempts have been made to make alloys in space, which is in a gravity-free state (e.g., M42 steel according to ASTM rule). The resulting alloys, however, are extremely expensive.
Catalytic agents formulated with rare metals were developed to improve such situations. For instance, 0.003% (30 ppm) of bromine element was added to a molten metal to remove P and S therefrom and ultimately to make CBM steel of high hardenability. Recent research for rare earth element combinations of lanthanum (La) and yttrium (Y) has allowed the making of special steel superior in wear resistance, impact resistance and toughness as well as reduced the amounts of conventional metal additives. This method, however, has such a disadvantage that the rare earth elements require the processes necessary for dressing and smelting.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide an additive composition for use in steel making, which is superior in deoxidation, desulphurization and dephosphorization and improves the fluidity of slag.
It is another object of the present invention to provide a method for making special steel superior in mechanical properties, including impact resistance, wear resistance and thermal resistance.
In accordance with an aspect of the present invention, there is provided an additive an additive composition for use in steel making, comprising: 84.8-99.3% by weight of an oxide component; 0.5-1.6% by weight of a metal component; and 0.04-0.07% by weight of a rare-earth element component.
In accordance with another aspect of the present invention, there is provided a method for making special steel, comprising the steps of. adding over four times, four aliquots of the additive composition comprising 84.8-99.3% by weight of an oxide component; 0.5-1.6% by weight of a metal component; and 0.04-0.07% by weight of a rare-earth element component at an amount of 15-25% by weight of a scrap iron base to be molten, in a blast furnace while the temperature is maintained at 1,600-1,700xc2x0 C.; removing slag from an iron melt in the furnace; and carburizing if the iron melt is short of carbon content.